Field: This invention is in the field of miniature devices that measure true viscosities of liquid.
State of the art: Viscosity is a measure of resistance of liquid to flow and its value depends on the rate of deformation for Non-Newtonian liquids as described in Dynamics of Polymeric Liquids, Vol. 1, 1987 authored by R. B. Bird, R. C. Armstrong, and O. Hassager. The rate of deformation is given by a shear rate in a unit of (time)−1. The viscosity measured at a known shear rate is “true” viscosity. The dependence of the true viscosity on shear rate is a viscosity curve which characterizes material and is an important factor to consider for efficient processing. But in many cases viscosity is measured under ill-defined test conditions so that shear rate can not be known or calculated. Under ill-defined conditions, the measured viscosity value is only “apparent”. Since the true viscosity is measured at a known shear rate, the true viscosity is universal whereas the apparent viscosity is not. Instead, the apparent viscosity depends on the measuring system. For example, as a common practice, a torque of a spindle immersed in a sea of test liquid is measured while the spindle is being rotated at a constant speed. In this case the torque value only yields an apparent viscosity since the test condition is ill defined and a shear rate is not known. At best, the apparent viscosity can be measured as a function of the rotational speed of the spindle. The rotational speed of the spindle can be, in fact, correlated with the shear rate only if a “constitutive equation” for the test liquid is known. However, a “constitutive equation” is not known for almost all Non-Newtonian liquids. Therefore, true viscosity can not be measured with ill-defined test conditions for most non-Newtonian liquids.
The methods that give only apparent viscosities have been developed and used for quality controls in manufacturing and material characterization. Various on-line viscometers have been designed for real time viscosity measurement. Prior art U.S. Pat. Nos. 5,317,908 (Fitzgerald et al.) and U.S. Pat. No. 4,878,378 (Harada) are concerned with systems that measure apparent viscosities for process controls. Prior art U.S. Pat. No. 6,393,898 (Hajduk et al.) describes a system that measures many test liquids simultaneously. These viscometers measure apparent viscosities. However, because of the non-universality of the apparent viscosity measurement, a correlation of the apparent viscosity of a specific sample measured with a specific method to the true viscosity has to be found separately, when desired. Fundamental development of formulations or materials requires a true viscosity measurement. Also, the design of processing equipments and accessories such as dies, molds, extrusion screws, etc., require knowledge of the true viscosity of the materials. However, the apparent viscosity measurement has been used for a quick test as an indication since it is easier and faster to measure and often more economical. The true viscosity is more difficult to get and can be only measured with a few types of instruments: rheometers and capillary viscometers. The rheometers impose a precise and known shear rate on test samples thereby measuring true viscosities. The rheometers are versatile and equipped to measure other properties. Therefore they are usually expensive. Usually large amounts of sample are required for viscosity measurement with rheometers. Also, the rheometers are not well suited for on-line applications. Circular capillary viscometers are another type of instrument that can measure apparent and true viscosities depending on whether a proper compensation is taken into account. The capillary viscometer needs a measurement of pressure drop along the capillary for determining viscosity. Since the capillary is circular, only the pressure at the entrance and exit can be measured. Because of this limitation, the capillary viscometer measures only apparent viscosity unless the entrance effect is corrected by using two different capillaries with different length to diameter ratios. However, the use of two capillaries makes the viscometer bulky and measurements time consuming. Capillary viscometers are shown in the prior art: for example in U.S. Pat. No. 6,575,019 (Larson); U.S. Pat. No 4,920,787 (Dual et al.); U.S. Pat. No. 4,916,678 (Johnson et al.); and U.S. Pat. No. 4,793,174 (Yau). Microfluidic viscometers are also disclosed in prior art: for example in U.S. Pat. No. 6,681,616 (Michael Spaid et al.); and 20030182991 (Michael Spaid et al.). Residence time of a marker in a fluidic channel is used to measure the viscosity, which is not a true viscosity unless the test liquid is Newtonian.
Rectangular slit viscometers relevant to the current invention are also used to measure the true viscosity and they are well described in Rheology in Polymer Processing, 1976, authored by C.D. Han. In these viscometers, test liquid flows inside of a rectangular slit flow channel and local pressures along the flow channel are measured with deployed pressure sensors for a given flow rate. In contrast to the capillary viscometer, the inside of the slit is flat so that pressures in the slit can be measured with pressure sensors mounted in the slit. The positions of the pressure sensors have to be sufficiently inside of the flow channel so that pressures of a fully developed flow are measured. From the pressure measurement, wall shear stress can be calculated. As the flow rate is varied, shear rate can be varied. From the measurement of wall shear stress at different shear rates, true viscosities are calculated using the well known Weissenberg-Rabinowitsch correction, which is much simpler than using two separate capillaries in case of using circular capillary viscometers. These viscosity measurements however are only simpler if the width of the flow channel is sufficiently larger than the depth of the flow channel. These slit viscometers need pumping systems for a precise control of the volumetric flow rate of test liquid. Frequently, the slit viscometers are used as an attachment to extruders as the liquids flow out of the extruders. In current practice, the pressure sensors are mounted individually to the plate flush enough to measure unperturbed pressures. However, it is very well known that a perturbation of flow significantly influences pressure measurement, particularly for viscoelastic non-Newtonian liquids. Any slight surface roughness due to the mounting of the pressure sensors may be a source of test sample deposition which degrades long term performance. Mounting of individual pressure sensors to eliminate surface roughness is difficult. Therefore, the measurement accuracy is often compromised depending on how well the individual pressure sensors are mounted in the flow channel. It has been found that the problems described above can be overcome with monolithically integrated pressure sensors in micro slit flow channels. With a single slit geometry, the shear rate can be only changed by the change of volumetric flow rate controlled by the pumping system. Most current slit viscometers are made individually with conventional machining processes, and are made for relatively large samples. Therefore, these conventional slit viscometers are not appropriate for measuring viscosities of test samples that are only available in a small quantity. Use of a micro slit flow channel with monolithically integrated pressure sensors can be tremendously advantageous. The micro slit viscometers allow the employment of microfabrication processes used to make micro chips and therefore these micro slit viscometers can be made in large quantity on a single wafer. This invention therefore makes the micro slit viscometers extremely cost-effective.